US-2001/0050067 A1 shows a VVT controller, which adjusts opening and closing timing of intake valves and exhaust valves in order to enhance the output of the engine, to improve the fuel efficient, and to reduce the emissions. The VVT controller has oil pressure control valves at the intake side and the exhaust side of the engine, and has an oil pump for supplying the oil pressure to the oil pressure control valves.
As shown in FIG. 16, the control mode of each VVT device is changed independently. For example, as shown in FIG. 17, when a target advance angle of the intake camshaft TAAI is under a threshold angle which is set around the most retarded angle, an actual angle of the intake camshaft AAI is maintained at the most retarded angle, which is referred to as a base mode, by controlling a duty signal DSI supplied to the intake-side oil pressure control valve IOV.
When the target advance angle of the intake camshaft is above the threshold angle, the intake camshaft is rotated so that an angle difference between the actual angle and the target angle becomes small, which is referred to as a feedback mode.
Similarly, the exhaust camshaft is adjusted to the base mode or the feedback mode based on whether or not the target angle of the exhaust valve TRAE is under a threshold angle which is set around the most advanced angle. AAE represents a target retard angle of the exhaust camshaft, and DSE represents an duty signal supplied to the exhaust-side oil pressure control valve EOV.
In such a VVT system in which the rotational phases of the intake camshaft and the exhaust camshaft are varied independently, when one of the intake camshaft and the exhaust camshaft is in the feedback mode, the other can be changed from the base mode to the feedback mode. In this case, the actual valve timing of the camshaft in the feedback mode is varied due to the mode changing of the other camshaft.
During the base mode, the cam shafts are kept at the most advanced angle or the most retarded angle, so that little oil flows between the VVT device and the oil pressure control valve. On the other hand, during the feedback mode, the oil flows from the oil pump to the VVT device through the oil pressure control valve. When the control mode of the intake camshaft or the exhaust camshaft is changed from the base mode to the feedback mode while the other camshaft is in feedback mode, the amount of oil being supplied to the other camshaft is varied. Thus, the actual valve timing of the other camshaft is fluctuated.
To avoid the fluctuation of the actual valve timing, two individual oil pumps can be provided for each camshaft. However, such a configuration needs large space and high cost.
Alternatively, the oil pipe from the oil pump can be divided into two branch pipes, each of which is connected to the oil pressure control valve. By increasing the volume of the branch pipes between the branch point and the oil pressure control valve, the fluctuation of oil pressure can be reduced. However, in such a configuration, the length of the oil pipe becomes long, which is against the low cost and the compactness.